(1) Field of the Invention
The present invention relates to a hightenacity conjugated fiber having excellent durability and suitable for industrial uses, especially as a rubber reinforcement. More particularly, the present invention relates to a conjugated fiber for industrial uses, which has excellent mechanical properties such as a high tenacity, a high modulus and improved dimensional stability, and improved adhesion to rubber, especially high-temperature adhesion, good heat resistance in high-temperature rubber, and improved fatigue resistance.
(2) Description of the Related Art
Polyester fibers represented by polyethylene terephthalate fibers are widely utilized for various industrial purposes because they are characterized by high tenacity and high elastic modulus, and these fibers are advantageously used as rubber reinforcements such as tire cords, transmission belts and conveyor belts.
Nevertheless, polyester fibers have poor heat resistance when embedded in rubber. Namely, at high temperature, ester bonds of polyester fibers are broken by the action of water or an amine compound contained in rubber, resulting in degradation of tenacity. Furthermore, polyester fibers have a poor adhesion to rubber, and when the polyester fibers are repeatedly exposed to high-temperature atmosphere for a long time, a problem such as drastic degradation of the adhesion to a rubber arises.
Tire cords composed of polyester fibers have been used in large quantities as carcass cords of radial tires for passenger cars, where the characteristics of high tenacity and high modulus are utilized. But when these tire cords are used for larger vehicles such as vans, trucks and buses, since the heat generated during running is easily accumulated in the tire, the tenacity is reduced by thermal degradation and the adhesion to rubber is lost, resulting in peeling. Accordingly, the heat resistance of polyesters in rubber must be improved to increase the adhesion at high temperature.
Many attempts to improve poor adhesion, a defect of polyester fibers, have been made, and in one of these attempts, a method was proposed in which the surface of polyester is covered with a polyamide. For example, Japanese Unexamined Patent Publication No. 49-85315 discloses a process for the preparation of a conjugated yarn comprising a polyester core and a nylon 6 sheath, in which the polymerization degrees of the constituent polymers and the ratio of the core polymer are specified and spinning is carried out by applying a non-humid lubricant and carrying out a direct spin-drawing. Furthermore, Japanese Unexamined Patent Publication No. 56-140128 discloses a rubber reinforcement composed of a sheath-core type conjugated fiber having a polyester core and a polyamide sheath, in which the ratio of the polyamide sheath component is 7 to 30% by weight and an epoxy adhesive is applied to the surface of the polyamide sheath component.
In the sheath-core type conjugated yarns proposed in Japanese Unexamined Patent Publication No. 49-85315 and Japanese Unexamined Patent Publication No. 56-140128, the adhesion to rubber is improved by the polyamide component as the sheath and the modulus or dimensional stability is maintained at a high level by the polyester component as the core. Namely, the adhesion is sufficiently improved according to this process, but the modulus and dimensional stability are degraded with an increase of the amount of the polyamide component as the sheath, and thus it is impossible to retain the satisfactory modulus and dimensional stability inherently possessed by the polyester fiber. Moreover, the heat resistance in rubber, the fatigue resistance, and other characteristics possessed by the polyamide component, are not sufficiently utilized.
Since the compatibility between an ordinary polyester such as polyethylene terephthalate and an ordinary polyamide such as nylon 6 or nylon 66 is poor if a conjugated fiber is prepared according to the usual spinning method, peeling or stripping often occurs at the polymer interface of the sheath-core conjugated structure, and the conjugated fiber does not have fatigue resistance sufficient for practical applications. Especially, the polymer interface is destroyed by the repeated elongation-compression fatigue undergone by the fibers at the drawing step, the tire cord-processing step such as the twisting or dipping step, the tire-curing step, and during running, and thus the required performances cannot be obtained from the sheath-core conjugated fiber.